Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. The engine knock may be addressed by retarding spark timing; however, significant spark retard can reduce fuel economy and limit maximum torque. Knock is especially problematic under boosted conditions due to high charge temperatures.
One method to reduce charge temperature and therefore knock, is via blowthrough wherein boosted intake air is blown through the combustion chamber to the exhaust during a positive valve overlap phase.
Another method to suppress knock is by diluting intake air with cooled exhaust gas recirculation (EGR). An example approach of controlling the flow of exhaust gases for EGR is shown by Roth (U.S. Pat. No. 8,495,992) wherein a split exhaust system separates exhaust gases exiting the combustion chamber during blowdown and scavenging phases. Exhaust gases from the blowdown phase are distributed either to the turbine in a turbocharger system or to an EGR system which directs cooled EGR gases to the intake manifold or upstream of the compressor in a turbocharger. Likewise, exhaust gases from the scavenging phase are conveyed to either an emission control device or to an EGR system which delivers cooled gases to the intake manifold or upstream of the compressor. Intake and exhaust valve timings are controlled to regulate the amount of exhaust gases flowing to the turbocharger and/or EGR based on engine operating conditions.
The inventors herein have identified potential issues, including issues with the above approaches to addressing knock limits. For example, an EGR throttle may be placed in the intake, upstream of the compressor, to enhance EGR flow at low backpressure which can make the turbocharger more sensitive to surge and increase pumping losses. Further, in the example where a blowthrough technique is used to reduce knock, additional fuel injected to bring exhaust gases to a stoichiometric ratio can cause over-temperature of the catalyst and affect emissions while increasing fuel consumption. Further still, engine efficiency may be degraded at lower engine loads and EGR may contribute to combustion instabilities.
The inventors herein have recognized the above issues and identified approaches to at least partly address the issues. In one example approach, a method for an engine comprises directing exhaust from a first cylinder group to one or more of a pre-compressor location, a post-compressor location, and an exhaust turbine, and directing exhaust from a second cylinder group to one or more of the pre-compressor location, and the exhaust turbine. In this way, exhaust gases can be recirculated by separate cylinder groups to distinct locations for improving performance and efficiency.
For example, a boosted engine may include a first cylinder group and a second cylinder group wherein the first cylinder group comprises cylinders distinct from the second cylinder group. Exhaust from the first cylinder group may be directed to one or more of three separate destinations including a first location upstream of a compressor (pre-compressor), a second location downstream of the compressor (post-compressor), and a third location directly upstream of an exhaust turbine. The second location downstream of the compressor may include a location downstream of an intake throttle and upstream of an intake manifold. Exhaust from the second cylinder group may be directed to one or more of the first location upstream of the compressor and the third location directly upstream of the exhaust turbine. As such, exhaust may be directed to one or more of the above described locations based on engine conditions. Exhaust from the first cylinder group may be directed to the second location during medium engine loads as well as lower engine loads while exhaust from the second cylinder group is concurrently directed to the exhaust turbine. During higher engine loads, a larger proportion of exhaust gases may be directed to the exhaust turbine from both the first cylinder group and the second cylinder group while directing a smaller proportion of exhaust gases to the location upstream of the compressor. Herein, the smaller proportion of exhaust gases may be blown through cylinders to upstream of the compressor along with fresh intake air by adjusting a valve timing to allow positive valve overlap between at least one intake valve and one exhaust valve of each cylinder of the first cylinder group and the second cylinder group.
In this way, knock can be reduced during different engine conditions while enhancing engine efficiency. Recirculation of exhaust gases from the first cylinder group to the location downstream of the compressor during specific engine conditions e.g. lower and medium engine loads, may enable a reduction in pumping losses as well as heat loss. At the same time, by directing exhaust from the second cylinder group to the exhaust turbine, desired engine power may be provided. As such, the reduced pumping losses and heat loss may improve engine efficiency. Further still, during higher engine loads, allowing fresh intake air to blow through any residual hot exhaust gases in the cylinders can lower temperatures within the combustion chambers. Moreover, since the blowthrough air is not directed to an emission control device, maintaining stoichiometric ratio in the exhaust with an injection of extra fuel may not be required.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.